The growing acceptance of medical and recreational marijuana means more people will be at risk for cannabis-induced psychosis, the researchers noted. They also said there is emerging evidence that marijuana use during adolescence may increase the risk of developing schizophrenia, a serious psychotic disorder.

The study of more than 700 people focused on the AKT1 gene. It's involved in dopamine signaling, which is known to be abnormal in psychosis. Dopamine is a neurotransmitter that plays an important role in mental health.

"We found that cannabis users who carry a particular variant in the AKT1 gene had a twofold increased probability of a psychotic disorder and this increased up to sevenfold if they used cannabis daily," wrote Dr. Marta Di Forti, of King's College London's Institute of Psychiatry in England, and colleagues. "Our findings help to explain why one cannabis user develops psychosis while his friends continue smoking without problems."

Psychosis is typically characterized by marked personality changes, impaired functioning, and a distorted sense of reality.

The findings, published Nov. 15 in the journal Biological Psychiatry, "could also help to design health educational campaigns tailored to reach those young people at particular risk," Di Forti said in a journal news release.

While identification of this gene variant will not lead to the development of a test to assess people's risk for marijuana-related psychosis, it does reveal a genetic factor that could help lead to new treatments for the problem, journal editor Dr. John Krystal noted in the news release.